Impedance coupled amplifier



June 23, 1936. A, C, T KER 2,045,316

IMPEDANCE COUPLED AMPLIFI ER Filed April 28, 1934 FREQUENCY arr-545,1 0) ,5 RANGE.

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Patented June 23, 1936 UNITED STATES IIVIPEDANCE COUPLED AMPLIFIER Arthur 0. Stocker, Haddon Heights, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application April 28, 1934, Serial No. 722,845

6 Claims.

The present inventon relates to resistancecapacity or impedance coupled amplifiers for amplifying signals in a relatively wide frequency band.

The invention relates particularly to resistance-capacity coupling networks for amplifiers wherein the signal may vary in frequency between relatively. wide audio frequency limits, such, for example, as from 20 cycles to one megacycle;

As is well known and understood, an amplifier of the above type includes one or more interstage coupling networks comprising an output or plate coupling resistor, an input or grid coupling resistor, and a coupling condenser. A coupling network of that character may be employed, for example, between two amplifier stages or between an amplifier stage and an output circuit such. as a transmission line.

In designing a resistance-capacity coupled amplifier for covering a wide frequency band, the coupling or grid blocking condenser becomes an important factor at both high and low frequency limits.

The low frequency portion of the frequency response characteristic of the coupling network,

embodying resistance-capacity coupling means, may be improved by increasing the size of the coupling condenser but not without greater increased cost. It is, therefore, an object of the invent-ion to provide means for increasing the low frequency response characteristic of a resistance-capacity coupled amplifier, without increasing the size of the coupling condenser.

It is a further object of the present invention to provide an improved coupling network for an electric discharge amplifier system embodying resistance-capacity coupling means which may have an improved overall frequency response characteristic and high gain.

The invention will, however, be better understood from the following description when considered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing,-

Figure 1 is a schematic circuit diagram of an electric discharge amplifier provided with an interstage coupling network embodying the invention;

Fig. 2 is a simplified equivalent circuit diagram of the amplifier shown in Figure 1 whereby the design and operation thereof may better be understood;

Fig. 3 is a vector diagram of voltages existing in the coupling network shown in Figs. 1 and 2; Fig. 4 is a curve diagram showing the effect of the coupling network of Figs. 1 and 2 upon a resistance-capacity coupling, network; and- Fig. 5 is a schematic circuit diagram of an amplifier showing a further embodiment of the invention.

Referring to Figure 1, l0 and ii are electric discharge amplifier devices or tubes of the high mu type in an amplifier having input terminals i2 and output terminals l3 and an interstage coupling network ii. The first stage amplifier device I0 is of the screen grid or other high gain type comprising a control grid IS, a cathode IS, a screen grid I1 and an output anode IS. The control grid I 5 is coupled to one of the input terminals i2 through a couplingcondenser l9 and is provided witha grid resistor 20 connected to ground 2i through a filter impedance 22. The cathode i6 is provided with a self-bias resistor 23 also connected to ground. A bypass condenser 24 is provided for the filter impedance 22 to cathode, thereby completing the input circuit of the first stage device.

The screen grid is supplied with positive potential from any suitable source (not shown) through a supply lead 25 and the output anode I 8 is likewise connected to a positive source of anode potential through an output circuit 26 in which is connected an output or anode coupling resistor 21 and a filter impedance or resistor 28. The filter resistor 28 is provided with a bypass condenser 29 connected to ground as indicated, thereby completing the output circuit of the first stage device ill.

The second stage amplifier device ii is likewise preferably of the high gain type and is provided with a cathode 30, a'control grid 3|, a screen grid 32, and an output anode 33, connected through an output circuit lead 34 to one of the output terminals. i3, the other of which is con-- nected with a positive potential supply lead 35 as indicated.

The input circuit for the second stage amplifier device Ii comprises an input grid lead 36 and a grid or input coupling resistor 31, terminating with a ground connection to cathode as indicated. Negative biasing potential is applied to the grid 3i through the resistor 31 from any suitable source such as the resistor 38 in the cathode circuit, as shown. The screen grid 32 receives a aagsitive biasing potential through a supply lead The output circuit of the first stage amplifier is coupled to the input circuit 01' thesecond stage amplifier through the coupling condenser shown at 40. Signals appearing across the output coupling impedance or resistor 21 are applied to the input coupling resistor 31 through the coupling condenser. For an amplifier designed to cover a wide frequency band, the coupling or grid blocking condenser 40 becomes an important factor in attaining high fidelity particularly at the low frequency limit of the audio frequency range. For a given second stage amplifier device II, the maximum value of grid resistance 31 or R as it will hereinafter be referred to, is fixed and the product CgRg which determines the grid circuit distortion at a given frequency may be increased by increasing the value of Cg, the latter being the capacity of the condenser 40 and hereinafter being referred to as Cg.

However,as the physical dimensions of Cg increase, a point is soon reached where an increase causes an appreciable rise in circuit capacity to ground, which capacity appears in parallel with the parallel resistance Hz: or the resistor 21 and By, and, for a given combination, determines the high frequency distortion. For a given ground capacity, the high frequency distortion may be reduced only by reducing the parallel resistance value usually by reducing Rp. This not only reduces the gain from the first stage amplifier device IO, but necessitates a larger filter condenser 29 or C; as it will hereinafter be referred to. Therefore, it is usually advantageous to maintain Cg as low in value as possible.

It has been determined that the distortion caused by a low product CgR 'may materially be lessened by utilizing a low filter capacity Cf. In fact, as a limiting case, by making infinite both the filter impedance 28 or R as it will hereinafter be referred to, and the internal plate resistance of the first stage amplifier device I 0 or Tp as it will hereinafter be referred to, the low frequency distortion may be made any value desired and, by making the product CgRq equal to CfRp, may be reduced to zero.

Referring now to Fig.2, the internal impedance of the first stage amplifier device In is indicated at T9 together with the re or the voltage generated by the tube I 0 connected in parallel with the output coupling impedance R1) and the bypass condenser C; in series, as one branch of the output circuit, the other branch of which is the coupling condenser Cg and the load or grid resistor Ry. The voltage existing across the load impedance or from the plate to ground is indicated as e; and the output voltage across the grid resistor Ry is indicated as ea. The currents flowing in the two plate or output circuits are indicated as i1 and i2 and the voltage drop through the internal tube impedance T1: is equal to the vectorial value of (il+'i2)Tp (1);ie =e1+(i1 +iz)r ==Voltage generated by the driver tube or first stage tube In, vectorial values being used throughout.

The voltages er and e1 will be out of phase with each other as indicated in Fig. 3, by the vector lines representing said voltages, the angle between the voltages being determined by the value of the condenser 40 and other circuit constants. For minimum low frequency distortion, the product CgRg must equal CjRp when impedances r, and R; are relatively high with respect to R9 and the reactance of C Furthermore, the voltage ilTp should be in phase with the voltage izr and v the voltage e2 should be in phase with the voltage (i1+i2)r as given in Equation (1) ab ve,

For minimum low frequency distortion, the product CgRq must equal C1Rp when impedances r, and. R; are relatively high with respect to R? and Ra, for by this means alone may the vector m, be made to lie parallel to the vector i2. Then, the vector in", being parallel to if, it, becomes parallel to 121', (actually parts of the same straight line). The generated voltage re, lies at an angle to 'ilTp and izr because of the voltage e1 (see Equation 1) but, since T1) is specified much larger than Rp, the generated voltage [Leg lies substantially parallel to 2 and therefore ez.

It isobvious that the product CgRg will vary with frequency but the product C/Rp will varyin exactly the same manner so that the products will remain equal. For that reason, the length of the vectors in Fig. 3 will vary with frequency but as they vary in identical manner and quantities, the shape of the vector diagram remains constant. Therefore, the ratio of ez to eg and the phase angle between the two are independent of frequency, thus fulfilling the requirements for distortionless amplification.

Vectorially adding the voltage, (i1+i2)r which is substantially parallel with the voltage e2, to the voltage e1, it will-be seen that voltage ,Lteg will lie substantially parallel, and, therefore, in phase with the voltage 62. In other words, the desired condition of operation is obtained when the output voltage 82 across the coupling network is in phase with the input voltage or voltage generated by the driver tube [.teg. This condition of operation depends upon the relation between the impedance in the branch circuits comprising in the one circuit, the output coupling or plate impedance Rp and the filter condenser Cr, and, in the other branch, the coupling condenser Cu and the input or grid resistor Ry. In the relation, as hereinbefore indicated, and as follows:

This equation may be derived from two equations as follows:

Capacitive reactance of condenser C1:

( 5) gi=gifirom vector diagram) This is the requirement for the two impedances which determine the angle of i1 and if to give 2'1 and is parallel in Fig. 3. or-- (6) KCaRv=KC1 2 where K=21rf A condition where the current in the two branch circuits are out of phase with each other and out of phase with the output voltage is indicated in the diagram in Fig. 3 by the dotted lines. Under such conditions, it will be seen that the voltage generated by the driver tube l0 resulting therefrom is widely out of phase with the output voltage e2 thereby causing appreciable distortion. For minimum distortion, therefore, the output voltage across the grid or load impedance must be in phase with the voltage generated by the driver tube, and the currents through the branch circuits of the network must also be in phase with each other and with the input voltage en.

With a circuit arranged in accordance with the Equation (2), the coupling condenser 40 may have a minimum capacity while, at the same time, the low frequency response characteristic of the amplifier network may be maintained at a relatively high level, the gain of the low frequency response being indicated in Fig. 4 by the solid line curve 42 as compared with the dotted curve 43 for a normal resistance capacity coupling circuit not including the frequency characteristic correction system shown and described.

It is assumed that r, and R; are greater than Rp or Rn. However, this is not a limiting condition. If either of the impedances are less in value, the vector #69 (Fig. 3) swings more to the right and out of phase with en. This may be corrected for by making C; still smaller, thus swinging ilTp to the left and bringing #67 back in phase with ex.

The swing to the right observed in e, is caused by the apparent change in capacity of C; if a small R} is used, or to shortening of the vectors ilTp and 271! if low r is used. In either case, a partial correction. may be obtained by reducing Cy,- thus swinging the vector #69 toward its desired position. i

In this case the shape of the vector diagram varies in such a manner that maximum distortionless amplification is obtained at the frequency at which the capacity reactance of C approaches the resistance Rf.

It will be appreciated that the response characteristic 42 is obtained without increasing the size of/the coupling condenser which is employed for obtaining a response characteristic as indicated by the curve 43 and the correction may be applied to any amplifier covering any predetermined frequency range, the zero point of the curves of Fig. 4 being the lower limit of the selected frequency range of operation. As determined by the use of an ordinary condenser as a, coupling means in the coupling network, the improvement in the frequency response characteristic is indicated in Fig. 4 by the added ranges A and B, the range B indicating the practical extent of the range to a point of permissible distortion. From an inspection of the curves, it will be seen that a relatively greater improvement in the low frequency response characteristic is obtained without added equipment or cost in providing the interstage coupling network and without impairing the high frequency response characteristic.

Referring to Fig. 5; an output coupling system for an amplifier is shown in connection with a resistance-capacity coupling network for a transmission line.

The amplifier or output stage comprises two parallel connected high impedance electric discharge amplifier devices 45 to which signals are applied through input terminals 46 and a coupled condenser 41 and grid impedance 48. Feedback preventing resistors 49 are provided in the grid leads and bias potential is applied to the grid input circuits from a suitable source such as a self-bias resistor 50 in the common cathode lead. The return circuits are grounded as indicated. The output circuits of the two amplifier devices 45 are connected in parallel to an output circuit 5i in which is located an output coupling resistor 52 or R4) and a filter impedance 53 or R] which, in the present example, is an audio frequency choke coil. In circuit with the impedance R; is a high frequency choke coil 54 and a current limiting resistor 55. The circuit is connected to a high voltage positive anode supply lead 56 along with a supply lead 5! for the screen electrodes for the devices 45.

-An output load circuit 58 is connected across output terminals 59 in turn connected through coupling condenser 50 and condenser 6| across the output coupling impedance Rp.

The output load circuit 58 is provided by a low impedance shielded transmission line which may be of the type known commercially as RCA "Cabloy", having relatively low impedance at signal frequencies. The line load may be considered as R while the coupling condenser 60 may be considered as C The condenser 6| is part of one branch circuit with Rp and may be considered as C].

With a pair of RCA-241 tubes located at 45 and with an output line 58, having an impedance of 50 ohms, R equal to 50 .ohms and coupling condenser Cg and filter condenser C each equal to 250 MF 9. maximum low frequency and high frequency response is obtainable. Without the use of the circuit shown, a coupling condenser at Cg of substantially 750 MB is required together with a lower impedance at Rp- Therefore, it will be seen that a material saving in cost is obtainable, together with an improved frequency characteristic through the use of coupling elements in a resistance-capacity coupling network embodying the invention. I

In the present example, the filter choke coil 53 and the plate impedance of the tubes 45 are preferably relatively high as shown and described in the preceding Figs. 1 and 2. Since the operation is the same for the coupling network of Fig. 5 as for the coupling network of Fig. 1, further description is believed to be unnecessary.

I claim as my invention:

1. A resistance-capacity coupling network including in combination an output anode circuit comprising branch circuits, one of said branch circuits including an output anode coupling resistor and a filter condenser in series relation to each other and to the anode circuit, and the other of said branch circuits including a coupling condenser and a load or grid resistor in series relation to each other,-the product of the capacity of the coupling condenser and the resistance of the load resistor being substantially equal to the product of the resistance of the first named output coupling resistor and the capacity of filter condenser.

2. In a resistance-capacity coupled amplifier, an interstage coupling network comprising in combination, an output anode resistor, a filter condenser in series therewith, and a second cir- 1 cult substantially in parallel with said resistor and condenser and comprising a coupling condenser and a coupling resistor, the product of the resistance and capacity values of which are equal to the product of the resistance and capacity values of the first named resistor and condenser.

3. A resistance-capacity coupled amplifier including an electric discharge amplifier device and an output coupling network therefor of the resistance-capacity coupling type, characterized by the fact that the coupling elements including a plate resistor R1), an output coupling resistor By, a coupling condenser Cg, and a filter condenser C; are of such related impedance and capacity values that the equation CgRg=RpcI is substantially satisfied, whereby the output voltage across the output coupling resistor is substantially in phase with the voltage generated by said device connected therewith.

4. An impedance coupled amplifier including a high gain amplifier tube, and an output coupling network therefor 01 the resistance-capacity coupling type, characterized by the fact that the coupling elements, including an anode circuit impedance device R a filter condenser C in series therewith, and a coupling condenser C, and load impedance H in series as a parallel circuit. for said first named impedance and condenser, are of such related impedance and capacity values that the equation C R =R C is substantially satisfied, whereby the output voltage across the load impedance is substantially in phase with the voltage generated by the amplifier tube connected therewith.

5. In an electric signal amplifier, a coupling network comprising a driver tube, an anode impedance therefor, a filter condenser in series with said impedance to signal currents, a filter impedance, and a coupling condenser and a load impedance connected in series, substantially in parallel with said series connected anode impedance and filter condenser, said coupling elements thereby forming two substantially parallel branch circuits having impedance values such that the product of anode impedance and the capacity of the filter condenser equals the product of the load impedance and the capacity of the coupling condenser, whereby the signal currents through said branch circuits are in phase with each other and in phase with the input voltage generated in said driver tube.

6. The combination with an audio frequency signal transmission line'having a relatively low audio frequency impedance, of amplifier means connected therewith to apply audio Irequency signals thereto in a low range of audio frequencies without appreciable distortion, said means including a plurality of amplifier tubes having anodes connected in parallel, an anode output impedance connected with said anodes, an anode filter impedance connected in series with said anode output impedance, a bypass capacitor connected between the junction of said impedances and a cathode return connection, a coupling capacitor connected between the anode and one side or said transmission line, and means providing a cathode return connection for the other side of the line, said anode output impedance and bypass capacitor providing a branch circuit from the anode to the cathode connection, and said coupling capacitor and transmission line impedance providing another branch circuit from the anode to the cathode connection, said branch circuits having such impedance values that the product of the anode output impedance and the capacity of the bypass capacitor equals the product of the transmission line impedance and the capacity of the coupling capacitor.

ARTHUR C STOCKER. 

